1. Field of the Invention
This invention relates generally to a system and method for improving the start-up reliability of a fuel cell system and, more particularly, to a system and method for improving the start-up reliability of a fuel cell system by providing a high frequency resistance measurement (HFR) of the fuel cell stack, where the system and method reduce cathode air compressor flow and/or provide a stack load for a stack current flow to reduce drying of the membrane and/or increase membrane humidification.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As is well understood in the art, fuel cell membranes operate with a certain relative humidity (RH) so that the ionic resistance across the membrane is low enough to effectively conduct protons. The relative humidity of the cathode outlet gas from the fuel cell stack is controlled to a desired relative humidity of the membranes by controlling several stack operating parameters, such as stack pressure, temperature, cathode stoichiometry and the relative humidity of the cathode air into the stack.
The end cells in a fuel cell stack typically have a different performance and sensitivity to operating conditions than the other cells in the stack. Particularly, the end cells are nearest in location to the stack's ambient temperature surroundings, and thus have a temperature gradient that causes them to operate at a lower temperature as a result of heat losses. Because the end cells are typically cooler than the rest of the cells in the stack, gaseous water more easily condenses into liquid water so that the end cells have a higher relative humidity, which causes water droplets to more readily form in the flow channels of the end cells. It is known in the art to heat the end cells of a fuel cell stack using resistive heaters positioned between the end unit and the unipolar plate so as to compensate for heat losses.
It has been shown that the longer a fuel cell system has been shut-down, the less reliable the next system start-up will be. Particularly, system start-up after the fuel cell system has been off for a significant period of time typically provides an occurrence of start-up failure as a result of one or more of the cells within the fuel cell stack lacking the ability to conduct the required amount of current. It has been suggested that one of the contributing factors for such a stack failure at system start-up is a result of the high resistance of the membrane within the fuel cell, which is a direct result of its ability to conduct the protons. It has been proposed that this high membrane resistance is a result of the membrane being to dry.